This invention relates to a high capacity energy storage device. The invention further relates to novel cathode materials.
It is well known that layered structures can be intercalated with small ions such as lithium, sodium and potassium. In particular, layered transition and main group metal chalcogenides have been used with lithium intercalant in electrochemical cells. It is recognized that increased loading capacity of the intercalant into the layered material would significantly improve device performance and battery capacity. Another limitation of the existing energy storage system using metal chalcogenides as the electroactive species is the strong dependence of the battery performance on the intercalant concentration, resulting in operation conditions changing with time. "Loading" or "loading capacity" is defined herein as moles of guest species per mole of host material; it should also be noted that "species" is defined to include both ions and uncharged atoms of the Group Ia elements.
Distortion of the metal chalcogenide layers has been used as a means of increasing the channel width and thereby increasing guest loading. U.S. Pat. No. 4,288,508 reports a cathode active material for a battery having the general formula Li.sub.z Na.sub.y TiS.sub.2, where y is preferable 0.15-0.20 and z can range as high as 3.25. Increased loading is accomplished by using the larger sodium atom to pry open the van der Waals channels. This results in significant distortion of the TiS.sub.2 lattice.
U.S. Pat. No. 4,309,491 reports a solid solution containing a bismuth chalcogenide for use as a cathode active material. The guest loading in the bismuth chalcogenide is reported to require up to six Faradays of electrons, suggesting that six moles of guest are intercalated. However, it is also reported that there is substantial dependence of the discharge voltage on the guest concentration.
It is the object of the present invention to provide a high capacity energy storage system whose performance is constant over time and changing intercalant concentration.
It is a further object of the present invention to provide an energy storage device including a metal chalcogenide cathode material capable of achieving high guest loading without the disadvantages of the prior art.